1. Field of the Invention
The present invention relates to an image heating apparatus such as a heat-fixing apparatus to be mounted on an image forming apparatus, for example, a copying machine and a printer, and an apparatus for reforming surface property of an image.
2. Related Background Art
As a heat-fixing apparatus that is one type of an image heating apparatus, those of several different systems are put to practical use. One of them is a heat-fixing apparatus of a heat roller system (FIG. 9) represented by a structure that includes a fixing roller 40 provided with a halogen heater 41 inside it and a pressure roller 50 forming a nip N with this fixing roller, nips and conveys a recording material P that bearing an image at the nip, and heats and fixes the image on the recording material. Another one is a heat-fixing apparatus of a film heating system (FIG. 10) represented by a structure that includes a heater 61 provided with a heating resistance layer on a ceramic substrate, a heat resistance film 63 moving while contacting this heater 61 and a pressure roller 53 forming a nip N with the heater 61 via the film 63.
In FIG. 9, reference numeral 44 denotes a thermistor for sensing a temperature of the fixing roller 40. In FIG. 10, reference numeral 64 denotes a thermistor for sensing a temperature of the heater 61 and 62 denotes a holder for holding the heater 61.
In particular, the heat-fixing apparatus of the film heating system has an advantage that a consumption power is small and a print waiting time is short because of a small heat capacity. Thus, the number of models of the image forming apparatus employing this heat-fixing apparatus is increasing.
Such a heat-fixing apparatus of the film heating system is proposed in Japanese Patent Application Laid-Open No. 63-313182, Japanese Patent Application Laid-Open No. 2-157878, Japanese Patent Application Laid-Open No. 4-44075 and Japanese Patent Application Laid-Open No. 4-204980.
Since the heat-fixing apparatus of the film heating system does not require energization of the heater in standby, it is possible to bring the heater to a heatable state by the time when a recording material reaches the heat-fixing apparatus even if the heater is energized after the image forming apparatus receives a print signal. Thus, the heat-fixing apparatus of the film heating system is an excellent heat-fixing apparatus that does not waste energy from the viewpoint of energy savings.
However, the heat-fixing apparatus using a fixing film has a structure in which a heat capacity is controlled as much as possible in order to satisfy quick-start property. Thus, it has poor thermal conductivity in its longitudinal direction and tends to keep a nonuniform temperature distribution. In particular, the heat-fixing apparatus of the film heating system has the following two problems a) and b). Each phenomenon will be described with reference to FIG. 12.
a) Initial Temperature Distribution
If a print operation is started from a state in which a temperature of the heat-fixing apparatus is sufficiently close to a room temperature, since the entire apparatus is cooled, although heat generated by energizing an energizing heating resistance layer of a heater warms a fixing nip portion, the heat is also released to an end portion in the longitudinal direction of the heater. Thus, as shown in FIG. 12, as an initial temperature distribution, a temperature is low at the end portion due to release of heat despite the fact that an uniform temperature distribution is maintained in the vicinity of the central position in the longitudinal direction (in the vicinity of 0 mm of the horizontal axis of the graph).
As a result, there is a problem in that, for example, a fixing performance of an end portion of a wide recording material is inferior to a fixing performance in the vicinity of its center if an unfixed toner image on the recording material is heat-fixed.
In order to avoid this problem, there is a method of enlarging a heating area of the heater to be wider than a width for conveying the recording material or setting a resistance of the energizing heating resistance layer at the end portion to be high to cause the end portion to generate more heat, thereby solving the problem. However, if the width of the energizing heating resistance layer is enlarged, there is another problem in that, for example, the size of the entire apparatus becomes larger.
In addition, if an area sticking out of the conveying area of the recording material is enlarged or a heating amount at the end portion is made larger, sufficient fixing performance is obtained up to the end portion in the initial period after printing is started. However, if toner images are continuously heat-fixed, a problem as described in b) below occurs.
b) Temperature Distribution at Continuous Fixing
Although a heat quantity generated by energizing an energizing heating resistance layer of a heater is given to a recording material via a fixing film, if toner images are continuously heat-fixed, a degree of temperature rising is different between an area where the recording material is conveyed and an area where the recording material is not conveyed.
In other words, in the area where the recording material is conveyed, heat generated in the energizing heating resistance layer is consumed to melt and fix a toner image on the recording material. On the other hand, in the area where the recording material is not conveyed, a pressure roller is directly heated and the heat generated in the energizing heating resistance layer is not consumed by the recording material, heat quantities are gradually accumulated and the end portion in the longitudinal direction where temperature is low in the initial temperature distribution as shown in FIG. 12 is also gradually heated. As a result, as in the temperature distribution at continuous paper feeding in FIG. 12, temperature is unusually raised at the end portion despite the fact that a temperature distribution is substantially fixed in the vicinity of the center of the heater as in the initial period.
In particular, if the energizing heating resistance layer is made longer than the conveying area of the recording material to enlarge the sticking-out area of the energizing heating resistance layer or a resistance distribution is given to the energizing heating resistance layer to increase the heating amount at the end portion, temperature rising at the end portion at the time of continuous heating becomes intense.
Moreover, when power to be consumed in the energizing heating resistance layer increases by speeding up an image forming apparatus, the temperature difference between the conveying area of the recording material and the non-conveying area of the recording material (non-sheet passing area) is more remarkable. That is, since an amount of the recording material capable of being subjected to heat-fixing in a fixed time increases following the speeding-up of an image forming apparatus, more applied power is required. As a result, particularly in accordance with the speeding-up, temperature rising in the non-sheet passing area becomes large.
The unusual temperature rising in the non-sheet passing area necessitates an improved heat resistance grade of a material in the area and is likely to cause problems such as deterioration of an internal surface of the fixing film and damaged stability of an electrical power supply in an electrode.
As described above, in the heat-fixing apparatus of the film heating system, a heat capacity is controlled to be as small as possible because the quick-start property is recognized to be more importance. Thus, heat conductivity in the longitudinal direction is poor and, due to a relationship between the area where a recording material is conveyed and the heating area of the energizing heating resistance layer, a) heat is insufficient at the end portion in the initial period and b) unusual temperature rising occurs at the end portion at the time of continuous heat-fixing. Thus, means has not been found so far which secures the quick-start property and attains both of the fixing performance at the end portion in the initial period and the prevention of temperature rising of the non-sheet passing area at the time of continuous heat-fixing apparatus. In addition, the above-mentioned problems are obstacles for speeding up the image forming apparatus.
The present invention has been devised in view of the above-mentioned drawbacks, and it is an object of the present invention to provide an image heating apparatus that can control (restrain) excessive temperature rising in a non-sheet passing area.
It is another object of the present invention to provide an image heating apparatus that can offset an insufficient heat quantity at an end portion thereof in an initial few sheets in continuously heating a plurality of sheets of recording materials.
It is still another object of the present invention is to provide an image heating apparatus, comprising: a heating member; a first heat generating element mounted on the heating member; and a second heat generating element mounted on the heating member, the second heat generating element having a resistance value per unit length at an end portion thereof which is larger than that at an end portion of the first heat generating element, wherein the first heat generating element is located on an upstream side of the second heat generating element in a moving direction of the recording material.
It is still another object of the present invention is to provide an image heating apparatus, comprising: a heating member; a first heat generating element mounted on the heating member, the first heat generating element having a resistance value per unit length which is substantially uniform in a longitudinal direction; and a second heat generating element mounted on the heating member, the second heat generating element having a resistance value per unit length which is nonuniform in the longitudinal direction, wherein the first heat generating element is located on an upstream side of the second heat generating element in a moving direction of the recording material.
It is still another object of the present invention is to provide an image heating apparatus, comprising: a heating member; a first heat generating element mounted on the heating member; and a second heat generating element mounted on the heating member, the second heat generating element being longer than the first heat generating element, wherein the first heat generating element is located on an upstream side of the second heat generating element in a moving direction of the recording material.
It is still another object of the present invention is to provide an image heating apparatus, comprising: a heating member having a heat generating element; and a heat releasing member being capable of contacting with and separating from an end portion of the heating member.
Still another object of the present invention will be apparent from the appended drawings and the following detailed description.
Embodiments of the present invention will be hereinafter described with reference to the accompanying drawings.